The so called “wireless” communicating systems which are more and more used daily and often of quasi permanent manner by an always increasing user population, all have antennas for receiving and more often for transmitting signals in the frequency band defined by the standard technology that regulates them. It mainly concerns mobile phones, particularly those based on the standard called GSM, standing for “global system for mobile communications”, which defines a communication standard that has worldwide geographical coverage.
Another very widely used communicating system, which requires a very sensible receiving antenna is the GPS, standing for “global positioning system”. By decoding signals from satellite network, this system in fact makes it possible to obtain, all over the terrestrial globe, a very precise geographic positioning of the receiver. GPS receivers are more often found in the mobile phones and in all kinds of so-called “smart phones” that also include all the functions of a personal digital assistant (PDA) and the ability to connect to the worldwide network of the Internet.
The wireless network may instead be conceived to only cover a limited geographic area, or even very limited as the so-called standard “Bluetooth” which allows communication up to around ten meters of terminals between them. Another very widely used communication standard of bigger range is the one called “Wifi” which allows to create a wireless local network or LAN, standing for “local area network”, in a limited geographic zone, frequently visited by the public: a building, the premises of the government or of a company, a café etc.
In spite of their miniaturization needed to fit the dimensional constraints imposed by the always smaller housings, especially when the thickness becomes very small, the antennas of the above devices have to be nevertheless able to maintain a maximum efficiency throughout the frequency bands where they are operated. Such efficiency depends on losses which are inherent to the antenna and which are most commonly measured using parameters called “S”, “scattering parameters” that enables qualifying the behavior of the antenna between the propagating medium on the one hand and the electronic control circuit on the other hand. Generally speaking the parameters S have been conceived and used to measure and qualify the behavior of linear passive or active circuits operating in the frequency range mentioned above often referred to as hyper frequencies (microwaves) or radio frequencies in the technical literature of these matters. It allows estimating the electrical properties of these circuits such as their gain, the loss in yield where the voltage standing wave ratio resulting from an impedance mismatch observed between the control circuit and the antenna. The matching of the antenna is particularly defined by the parameter S11 representing the reflection loss of the antenna. It is expressed in decibels (dB). The lower the value of S11 is the better the matching and thus the overall efficiency of the antenna.
The parameter S11, which is frequency dependent, allows defining the bandwidth of the antenna, that is, the frequency band in which S11 remains less than a given threshold, which is typically defined at a level of −6 dB. Under these conditions, one quarter of the power delivered by the control electronic circuit is lost by reflection, and three quarters are thus usefully radiated by the antenna.
The bandwidth of an antenna can be more or less wide. It is often expressed in percentage of its central frequency. An antenna whose bandwidth is of a few percentages is considered to have a narrow operating band. This type of antenna is suitable for certain applications. For example, for a GPS receiver, an antenna whose bandwidth is of the order of 2% is sufficient.
An antenna whose bandwidth is equal or larger than 15% is considered to have a wide operating band. Those whose bandwidth are larger than or equal to 20% have a very wide bandwidth. It is to be noted here that to describe this type of antenna, the acronym of “UWB”, “ultra-wide band” is also often used.
The use of a very wide band antenna potentially offers many advantages. A single broadband antenna can thus simultaneously cover multiple radiofrequency standards. This reduces the number of antennas that has to be implanted in the multiservice wireless devices such as the smart phones, which not only gives an advantage in terms of cost but also makes it possible to overcome technical problems otherwise difficult to solve such as parasite couplings that may occur between the different antennas of the same smart phone.
Furthermore, the development of applications requiring to be able to download and transmit always larger quantity of data, notably the transmission of video signals, led the standardization bodies to define communication protocols offering wider and wider bandwidths. For example, in 2002 frequency bands ranging from 3.1 to 10.6 GHz to so-called UWB standard were allocated (in the form of six groups representing fourteen frequency bands, with 528 MHz of width each), for short-distance communications of WiFi type. The emergence of UWB-based communication applications is remarkable, which contributed to highlight the need for very wide band antenna solutions, which should be readily industrialized, inexpensive and easy to integrate.
However, the realization of broadband miniature antennas faces considerable theoretical and technical problems. It is particularly well known that obtaining small-sized antennas in view of the wavelengths to be transmitted can only be done at the price of dramatically reducing their bandwidth, which goes directly against the purpose.
It is therefore an object of the invention to provide a solution to this problem by reducing the size of the antenna intended to be implanted in the same housing while their control circuit can still maintain sufficiently wide bandwidth for operating.
Other objects, features and advantages of the present invention will appear when studying the following description and accompanying drawings. It is understood that other advantages may be incorporated.